`nullptr` initialization of pointer member variables in class definition

Nobody has claimed this yet.

Currently, mgmol classes do not initialize pointer member variables in their class definitions. For example of `MGMol`,

```
template
class MGmol : public MGmolInterface
{
private:
std::ostream& os_;

MPI_Comm comm_;

XConGrid* xcongrid_; // <--

OrbitalsType* current_orbitals_; // <--

AOMMprojector* aomm_; // <--

Ions* ions_; // <--

....
```

This is very error-prone and hard to debug, as they have uninitialized pointers which exhibit unexpected behaviors. This practice should be changed as soon as possible, as below:
```
template
class MGmol : public MGmolInterface
{
private:
std::ostream& os_;

MPI_Comm comm_;

XConGrid* xcongrid_ = nullptr;

OrbitalsType* current_orbitals_ = nullptr;

AOMMprojector* aomm_ = nullptr;

Ions* ions_ = nullptr;

....
```

This is usually the safest and shortest initialization. An alternative is to define them as `nullptr` in the constructor (For example, `MGmol::MGmol(...)`).

Start by locating the MGmol class definition and its MGmol::MGmol(...) constructor, which the issue uses as examples. Inspect the pointer member declarations across the mgmol classes and identify which lack initialization. Done means the relevant pointer members are initialized to nullptr consistently; no tests or specific files are named in the issue.

Written by the indexing model from the issue text.